This relates to ambient sensing electrodes and in particular to electrodes that are adapted for operation in high temperature environments and to structures for mounting such electrodes.
It frequently is necessary to monitor the composition of a chemical environment, for example, to regulate chemical or biochemical processes, to determine air or water quality, or to measure parameters of interest in biomedical, agricultural or animal husbandry disciplines. One means for the detection, measurement and monitoring of the chemical properties of a substance involves the measurement of potential difference between two electrodes with the potential difference being dependent upon the chemical activity being measured. Because of the nature of the chemical environment, it is desirable that any measurement apparatus have at least some of the properties of: low cost, simple fabrication methodology, digital operation, some degree of signal preconditioning or intelligence, small size, high chemical sensitivity with selectivity, multiple species information with specificity, choice of reversible or integrating response to chemical species, temperature insensitivity or compensation and low power operation. In addition the measurement apparatus should have good long term electrochemical stability, good physical resiliency and strength and good resistance to corrosion and chemical attack. In the case of electrical measurement devices, the devices should also have low electrical impedance to provide good signal to noise ratio and preferably a Nernstian response to the chemical phenomena being measured.
Bergveld has proposed that hydrogen and sodium ion activities in an aqueous solution be measured by a metal oxide semiconductor field-effect transistor (MOSFET) modified by removal of the gate metal. P. Bergveld, "Development, Operation, and Application of the Ion-Sensitive Field-Effect Transistor as a Tool for Electrophysiology" IEEE Transactions of Biomedical Engineering, Vol. BME-19, pages 342-351 (September, 1972). In particular, if a MOSFET with no gate metal were placed in an aqueous solution, Bergveld suggested that the silicon dioxide insulation layer would become hydrated and then, because of impurities in the hydrated layer, ion selective. After hydration of the insulation layer of the MOSFET, Bergveld believed the device could be used for ion activity measurement by immersing the device in the solution in question and then recording conductivity changes of the device. Thus, the Bergveld device is commonly referred to as an ion-sensitive field effect transistor (ISFET).
Bergveld's work led to other developments in the field of ion sensitive electrodes such as the chemical sensitive field effect transistor (CHEMFET) device described in U.S. Pat. No. 4,020,830 which is incorporated herein by reference. As described in the '830 patent, the CHEMFET is a MOSFET in which the gate metal has been replaced by a chemically selective system that is adapted to interact with certain substances to which the system is exposed. Thus as shown in FIGS. 1 and 2 of the '830 patent, the CHEMFET is identical in structure to a MOSFET except for a membrane 38 that is deposited in place of a metal gate layer on the oxide insulator above the channel region of the transistor and, optionally, an impervious layer 44 that covers all other parts of the CHEMFET that might be exposed to the solution. Numerous variations on CHEMFET structures are disclosed, for example, in U.S. Pat. Nos. 4,180,771, 4,218,298, 4,232,326, 4,238,757, 4,305,802, 4,332,658, 4,354,308, 4,485,274 and 4,397,714. Further improvements in these structures are disclosed in Application Ser. No. 441,902.
Despite this intense development of new designs, there is still considerable work to be done to achieve some of the desirable transducer properties described above. One area in which improved electrodes are needed is the field of high temperature sensing. High temperature environments are typically extremely hostile to electrodes, causing the electrodes to deteriorate so fast that they cannot practically be used in many environments. For example, it is extremely useful to be able to sense ambient conditions in a borehole such as an oil well. However, these environments are often extremely hot and possibly corrosive as well. While information about the chemistry of this environment would be extremely desirable, the cost of obtaining it with conventional electrodes, which cannot survive in this environment for any period of time, cannot be justified.